TOC is paragraph driven. If you want the author to follow the title on the same line it needs to be part of the same paragraph. No doubt you Don't want them on the same line inthe body of the book, however, so this is going to be a case where there will be work to do, either in the body or TOC, or both, to get the lines split in one place, but not the other.

In the body of the book you can use a forced line break, but that will carry into the TOC any will need to be removed there each time the TOC is updated, or you can try a strategy of adjusting frame widths and or paragraph indents (coupled with No Break or non-breaking spaces judiciously used, perhaps) to force the lines to break as you like, which will not show up in the TOC, and may not necessarily work in the layout, depending on how you want the two lines to appear, which we don't know. Ther is the further complication here of needing a Tab someplace to act as separator foor the TOC, or you are back to adding it every time the TOC is updated.

Another complication with having two lines in a single paragraph is you probably want the formatting diiferent for the title and author. You can do that by applying character styles, but if you don't want those styles to carry through to the TOC they need to be applied as nested styles in your paragraph style definition. Manually applied styles or local formatting overrides will appear in the TOC even if you change the underlying style for the entry.

What ALWAYS works is to add a new style to use for the TOC to pick up and use it to set a paragraph that says exactly what you want to appear in your TOC listing. I would set this in a separate frame that can then be anchored in the text in case the pagination changes during editing, and inthe attributes panel you should set this frame to non-printing so that it shows on screen, but will not output. I generall make these as obvious as possible on screen, like using red type, so I don't forget they are there.